NOTE: The muscles used here are also very close to the sexual organs, and until you learn to isolate and strengthen the pelvic floor (perineum) muscles, you will also be contracting musculature close to the sexual organs. For many people, this can bring up sexual energy. If this happens for you, relax the body and release any tension, then do a few rounds of Kapalabhati breathing. This will help dissipate some of that energy. Also, focus on drawing that energy up the spinal column into your head and your heart. Visualization is a powerful tool, and consciously guiding this energy up the body can help give you greater mental and physical energy in the body. I recommend practicing these techniques in a location where the release of sexual energy will not cause any problems for you, until you learn to isolate the muscles without releasing that energy in an uncontrolled way.

Mulabhanda (also known as the Root Lock or Core Lift). This lock increases and holds energy in the first and second chakras, channeling dormant energy from the base of the spine. It contracts the rectum, sexual organs, and navel. This is similar to a Keggle exercise, for those who are familiar with that technique. 

To engage Mulabhanda, inhale deeply. As you exhale, contract and draw in the anal sphincter muscles, as though you were holding in a bowel movement. Then contract the muscles of the urethra, as though you needed to urinate and had to hold it in. Finally, contract your abdominal muscles in, pulling the navel back towards your spine. Some people find it useful to contract the muscles 1 inch below the navel and 2 inches into the body.  At first, you will be contracting the muscles of the buttocks as you learn this technique.  Eventually, you will be able to isolate the perineum (muscles of the pelvic floor between the anus and the genitals, which holds your organs inside the pelvis).  Once you can isolate the perineal muscles, you will be able to lift just the center of the perineum and rather than a large contraction pulling the whole floor up and into your body, you will be able to use a subtle contraction, lifting just the center of the perineum like a gentle pinch lifting a tissue.

Inhale, release, exhale, engage. To strengthen the muscles, hold the root lock as long as possible before releasing. Eventually, you will be able to hold it on both the inhalation and exhalation by isolating the muscles of the perineum (pelvic floor), but at first, it will probably move with your breath.